1. Field of the Invention
The present invention relates to irreversible circuit elements, particularly irreversible circuit elements preferably for use in microwave bands, such as isolators, circulators, and the like, and also relates to modules provided including such irreversible circuit elements.
2. Description of the Related Art
In general, conventional irreversible circuit elements such as isolators, circulators, and the like have characteristics that signals are transmitted only in a predetermined specific direction and not transmitted in the reverse direction. Isolators, for example, are used in transmission circuits of mobile communication devices such as cellular phones and the like while making use of the above characteristics.
Recently, it has become possible for a single cellular phone to carry out communication operation in a plurality of different frequency bands. In order to implement this function, Japanese Unexamined Patent Application Publication No. 2002-517930 proposes a power amplification module for a dual mode digital system in which two transmission output portions are connected to an antenna through a diplexer.
However, in the proposed module, a tuner is needed to be provided, in addition to the diplexer, between the diplexer and the antenna for impedance matching in order to support a plurality of frequency bands. This increases the number of components, costs, and so on. Further, the proposed module has a problem that load fluctuation (impedance fluctuation) on the antenna side directly gives unfavorable influence on the transmission circuits.